This invention relates generally to the aeration of wastewater and more particularly to an air diffuser membrane that is treated with a biocidal agent which inhibits biological growth on the membrane surface.
Flexible membrane diffusers are in widespread use for the aeration and mixing of wastewater. By way of example, U.S. Pat. No. 4,960,546 to Tharp discloses a flexible membrane diffuser that is sleeved onto a rigid tubular diffuser body to provide a construction that has achieved considerable popularity. The membrane collapses on the diffuser body and fits closely around it when no air is being supplied by the blower to the piping of the aeration system. When the aeration is active, air is supplied through the piping to the diffuser body and enters the inside of the membrane through ports in the diffuser body. The air pressure causes the membrane to expand as it receives the air.
This type of diffuser membrane is provided with a large number of aeration slits which are closed when the membrane is collapsed on the diffuser tube in the absence of air pressure. The wastewater cannot seep into the aeration piping because the closed slits do not provide access and the membrane is impervious to liquid. However, when the air is turned on and the membrane expands, the slits open and allow the air to discharge into the wastewater in the form of fine bubbles. Such small bubbles are advantageous from an efficiency standpoint because their relatively small volume to surface area ratio results in more efficient aeration. The slits are controlled apertures because the extent to which they open varies with varying air pressure.
In addition to use in tubular diffuser systems, flexible membranes have been used in other diffuser configurations, including disk type diffusers. The membrane is used in this application as a flat disk member which covers the diffuser plenum in the body of the diffuser and discharges air in fine bubbles in much the same manner as in the case of tubular diffusers.
Flexible membrane diffusers are typically constructed of EPDM rubber or urethane. These materials exhibit the necessary structural characteristics. They are also flexible and elastic and thus able to meet the operational requirements for the membrane in these aspects.
Although flexible membrane diffusers have been satisfactory for the most part, they can be susceptible to biological fouling, especially when used in relatively severe applications where biological growth is promoted due to the conditions of the wastewater. In harsh wastewater environments, it is not uncommon for algae and other biological material to build up on the surface of the membrane, often quickly and to a significant extent. When biological growth accumulates on the membrane, there is a marked increase in the size of the bubbles that are released into the wastewater, as the fine bubbles tend to merge in the biological growth and create larger bubbles which are finally released into the liquid. As a result, the efficiency of the oxygen transfer to the wastewater suffers a significant reduction.
Although there have been proposals made to apply biocides to other types of membranes, such as filtration membranes which are permeable, impermeable flexible membrane diffusers have not been treated with biocidal agents. U.S. Pat. No. 5,102,547 to Waite et al. discloses a filter membrane that is treated with biostatic or biocidal agents. Because the membrane serves as a filter, it is necessarily permeable to liquids in order to allow liquids to pass through it for filtration of the liquid. This type of filter membrane is clearly of no use in treating wastewater and can only be used in clean water environments because it would quickly become fouled by solids in wastewater or other liquids having suspended solids. Similarly, U.S. Pat. No. 5,106,267 to Vaughn et al. is directed only to the biocidal treatment of porous or permeable membranes that have no applicability for aeration and are not useful in wastewater applications.
The present invention is directed to a flexible and elastic diffuser membrane which is used in the aeration of wastewater and which is impregnated or otherwise treated with biocide in order to resist biological accumulations on the membrane surface.
In accordance with the invention, a diffuser membrane is preferably constructed of a material such as EPDM rubber or urethane. The structural materials of the membrane are preferably mixed with a biocide such as carbolic acid which is thereby dispersed throughout the membrane structure. Alternatively, the membrane can be coated with biocide on the surface which is exposed to wastewater when the membrane is in service submerged in the wastewater.
In either case, the membrane is a flexible and elastic structure which is slitted to provide controlled apertures for the release of fine bubbles of air into the wastewater for aeration and mixing of the wastewater. The membrane may be closely sleeved on a rigid tubular diffuser body which is ported in order to apply air to the inside of the membrane. In the absence of air pressure, the membrane collapses tightly on the diffuser body and the slits are closed to prevent entry of wastewater into the piping of the aeration system. When air is applied, it expands the membrane and opens the slits to allow the discharge of air into the wastewater in the form of fine bubbles.
The membrane may take other forms, including the form of a disk diffuser in which a plenum is covered by a flat membrane. When air is supplied to the plenum, the membrane expands and the slits open for discharge of the air into the wastewater.
The treatment of the diffuser membrane with biocide inhibits the growth and accumulation of algae and other biological substances on the membrane. This has the beneficial effect of preventing biological growth from causing the air bubbles to increase in size, often dramatically, as can occur in wastewaters that have severe conditions conducive to biological growth. Consequently, the biocidally treated membrane of the present invention retains its ability to produce fine bubbles and thus continues to operate near peak efficiency even in harsh or severe wastewater conditions.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.